Light upconversion (UC) is a process capable of transforming low-energy radiation into blue-shifted light by combining the energy of two or multiple photons. Among the various upconversion-schemes, UC by means of triplet-triplet annihilation (TTA) relies on organic and/or organometallic dyes, which enable upconversion to occur at low radiation intensities (often at power densities of 100 mW·cm−2 or even lower) therefore widening the scope of UC-materials.1,2 TTA-UC systems usually comprise two dyes: a sensitizer, which harvests light and converts it into triplet excited-states and an emitter, which accepts and transforms these triplet excitons into high-energy radiation by means of TTA (FIG. 1). Important requirements for TTA-UC systems are shielding from oxygen, as it efficiently quenches excited triplet states3 and sufficient exciton mobility for both triplet-energy transfer and TTA-steps. Both requirements are best met in oxygen-free solutions.
TTA-UC has been proposed for energy-conversion related applications such as molecular energy storage,4-6 photoelectrochemical water splitting7-9 or soft actuators10 and for other applications such as bioimaging11,12 or oxygen sensing.13 In the last years, research efforts have also been directed towards the efficient implementation of TTA-UC in solid-state materials as they are more suitable for or even instrumental for certain applications. Organic materials only consisting of chromophores have been among the first solid TTA-UC materials studied14,15 and are still an important subject of investigation.16-18 Self-standing solid-state materials have been obtained by blending TTA-UC dye-pairs into polymeric19 and molecular gels20-22 as well as in rubbery23,24 or glassy18,25-28 polymer matrices. Usually, high dye contents exceeding 20 wt. % are required in rigid host polymers to compensate for the low translational and rotational mobility of the dye molecules and in order to maximize their upconversion efficiency.26-29 This can be rationalized by the fact that triplet-energy transfer from sensitizers to emitters mainly follows Dexter energy-transfer scheme30 and therefore requires close proximity (typically a distance of less than ca. 30 Å) of the moieties involved.31 In contrast to glassy polymers, rubbery polymers achieve high upconversion quantum efficiencies also at relatively low dye loadings (ca. 0.1 wt. % emitter-content). However, TTA-UC elastomers can suffer from phase segregation of the dyes blended therein and display more limited mechanical properties compared to glasses.23,24 A different strategy pursued to preserve the photophysical solution-properties of dyes in rigid materials is the use of rigid-shell, liquid-core capsules. Such capsules contain the TTA-UC dyes in their core and can subsequently be embedded in a polymer poly(vinyl alcohol)32 or cellulose nanofibers33 matrix, for example by electrospinning or solution casting. Unfortunately, such prior-art materials can only be fabricated by complex multi-step processes. Another problem is the fact that such approaches often lead to materials in which either the liquid-filled particles, their aggregates or the matrix cause scattering. This renders materials made with prior art processes often opaque, which in the context of the desired upconversion is undesirable. Opaque materials lead to a reduced light absorption and give lower quantum efficiencies.34 